Dunno if this qualifies, but I've been developing a mod I call Rise of Blockland.
In it, players are challenged to be the most successful and rich people possible in order to get their share of the brick pie. Despite the fact that there are no anti-spam rules whatsoever, because players are held accountable for each and every brick, the average build turns out better, in my opinion, than a typical server.
Players being forced to build next to each other, to advertise their products, to present an appealing form of business that will attract customers, to having builds accessible with a lack of jets or vehicles, etc. all create emergent construction styles and unique gameplay elements for how players approach the game and each other.
